1. Technical Field
The present invention is related to an antenna system to operate on different radio frequencies. Particularly, the present invention is related to an antenna system for GSM and WLAN radio operation.
2. Discussion of Related Art
Mobile communication techniques have brought and will bring to the marketplace new, versatile communication models in which new requirements are placed on multi frequency antenna assemblies not only operating in the mobile communication network according to the different radio frequencies of the Global System for Mobile communication (GSM) but also operating on other wireless communication standards and therefore other frequencies, for example wireless local area network (WLAN) according to WLAN 802.11b standard. Both GSM and WLAN communication are used for wireless data communication, wherein so called enhanced GSM (E-GSM) standard has been mentioned comprising GSM packet radio system (GPRS), high speed circuit switched data HSCSD or further enhanced data rate for GSM evolution (EDGE). A related mobile communication system according to GSM has additionally to be taken into account which is called Global Digital System for mobile communication (DCS).
GSM based mobile communication operates on different GSM frequencies wherein 900 MHz and 1.8 GHz are generally the two important ones but the related DCS communication frequency at 1.9 GHz has also to be considered. Antennas able to operate on the different GSM communication frequencies are realised and available.
Documents EP 0 923 158 and EP 0 938 158 shall only be referenced as the documents disclose multi resonant frequency antennas employable for GSM communication. The antennas are not suitable for operating on the ISM frequency band.
WLAN 802.11b mobile communication is a wireless communication standard to cover local areas to supply wireless access to a local area network, e.g., of a company. This standard uses a radio frequency at 2.4 GHz utilising the so-called industrial, scientific and medical (ISM) band. Generally, WLAN communication requires a system of two antennas, so called diversity antenna system to improve above all the receiving performance. The operating wavelength of WLAN communication is sufficiently short to accommodate conveniently small geometries available in planar antenna elements. In WLAN applications, an indoor environment is typically encountered wherein the propagation of radio waves from one point to another can be greatly affected by the surrounding structures of office areas, and activities taking place within the office area. Due to these effects diversity antennas are used to overcome the problem. Diversity antennas are distinguished by antennas placed in different and favourable positions often orientated in different directions, preferably arranged perpendicularly to each other.
Antennas for operating wireless communication according to the above mentioned wireless communication standard are state of the art and implemented in several devices in a great variety of designs. To provide a dual mode device able to use GSM data communication and WLAN data communication it would be an easy and fast way to combine devices according to the different standards. But this way of implementation is limited due to the fact that implementation of an E-GSM/DCS antenna and a WLAN/ISM diversity antenna system comprising two additional antennas is not possible since wireless communication is generally used in mobile devices which are restricted in size and weight. Additionally, the usage of two completely independent units for E-GSM/DCS communication and WLAN communication is cost-intensive, has an undesired high electrical power consumption and an undesired shape.
Particularly, the design of an interface card according to the personal computer memory card international association (PCMCIA) or the following interface standard distinguished as PCCARD used preferably in mobile terminals like mobile computers, personal digital assistant terminals (PDA) or the like puts high demands on size, shape, power consumption, mechanical durability and costs so that the implementation of two radio systems is not reliable.
The document EP 1 083 622 describes a switching circuit for multi-transceivers. The switching circuit allows to connect a plurality of transceivers to a antenna system operating in different operation modes. Herein, the antenna system consisting of two antennas. A first operation mode operates one transceiver in combination with one antenna of the system, a second operation mode operates a second transceiver in combination with the antenna system as a diversity antenna system and in a third operation mode the allows to operate one transceiver in combination with one antenna and the other transceiver in combination with the other antenna of the antenna system. The switching circuit is designed to operate transceivers transceiving on the same radio frequency band, herein particularly the 2.4 GHz ISM (industrial, scientific and medical) band. For that reason, the antennas of the antenna system coupled to the antenna switching circuit are identically and only designed for operating on a common resonant frequency. A combination of different antennas providing different resonant frequency is not intended. Further, the switching circuit does not provide a common production test RF interface in order to test and tune the transceivers.
Further, the document EP 0 623 967 describes a dual antenna diversity system for WLAN consisting of two L-shaped PIFA antennas. In that disclosure a single pole double throw switch (SPDT) is employed to effect the switching between the two antennas in a receive mode and for switching to only one of the two antennas for operation of the apparatus in a transmit mode. The respective switching is designed for operating only a WLAN transceiver and not designed with regard to the operation of a plurality of transceivers sharing the antenna system.
Moreover, the document EP 0 866 588 describes also a antenna switch and filter arrangement to operate a single transceiver. Again, no antenna sharing of a diversity antenna system is presented. The operation of WLAN transceivers in combination with diversity antenna system is state of the art and very popular, especially for operating WLAN interface adapters according to the PCCARD or PCMCIA standard.